Depressurization system for pressurized steam operated plant

ABSTRACT

A depressurization system (1) for pressurized steam operated plant employing injection of cold water contained in a superelevated tank (5), said injection being triggered rapidly and in a manner independent of the magnitude of the damage and of the presence of incondensible gases, by means of a natural circulation of steam in a closed loop of pipes (19, 20, 21, 22) around said plant (2, 3, 4), caused by the condensation of steam in a condenser (10), placed on said pipes, during the occurence of an accident, comprises a first siphon-type hydraulic shutoff (32), on said pipe loop (19, 20, 21, 22) and arranged close to the inlet collector (14) of said condenser (10), a second siphon-type hydraulic shutoff (29), on the cold water delivery pipe (26) and placed close to the pressurized steam operated plant (2, 3, 4), said second siphon-type hydraulic shutoff (29) being below said first siphon-type hydraulic shutoff (32).

DESCRIPTION

The present invention relates to a system for depressurizing pressurizedsteam operated plant.

The system is of the type in which cold water contained in a tank whichis superelevated with respect to the pressurized plant, is injectedunder gravity.

As is known, this type of system is used to guarantee maximum safety inthe event that pressurized plant suffer accidents which require theimmediate depressurization of the plant.

A typical application is in the field of water-cooled nuclear reactorsin which depressurization, in the event of a serious accident, consistsin injecting directly into the container of the nuclear reactor or intothe primary circuit, a large quantity of cold water which condenses thesteam present and lowers the average temperature of the plant, with aconsequent drop in the internal pressure.

Following this depressurization it will then be possible to bring inother accident mitigating systems.

An essential requirement in this sphere is that the depressurizationsystem should come into action without any possibility of failure, whichwould have devastating effects because said other mitigating systemswould be ineffective.

In this respect depressurization systems are known which employinjection, under gravity, of liquid coolant operating passively, i.e.able to fulfil their functions solely on the basis of physical laws, byvirtue of suitable mechanical/functional and structural design.

In the systems mentioned above, the injection of liquid coolant is nottriggered through the use of automatic logic enablers, by controlsystems, or manual ones, by operators. This therefore does not requirethe intervention of special actuation members such as, for example,valves, nor the presence of sources of external energy.

Patent Application No. WO 95/09425 describes a depressurization systemin which the injection of water is triggered by means of a pressurereduction induced in the throat of an ejector traversed by steam fromthe plant to be depressurized condensing in a condenser forming part ofthe depressurization system.

This condenser, during normal operation of the plant, is flooded bywater from the plant, whereas, should an accident occur, this condenseris gradually uncovered by the evaporation of the water, becomingoperational and bringing about said circulation of steam.

The depressurization system described above has the drawback oftriggering the injection of cold water at a speed proportional to therate at which the plant is draining. In particular, in the case ofaccidents which involve ruptures in the plant of limited flow section,the triggering rate is too slow.

In addition to a delay in the triggering of the injection of cold water,the aforesaid system has the drawback of suffering from the presence ofincondensibles in the steam, their presence further slowing down thecondensation of the steam to the point where triggering is precluded.

The technical problem underlying the present invention is how to provideavailable a depressurization system for pressurized plant such as toovercome the drawbacks mentioned with reference to the prior art.

This problem is solved by a depressurization system for pressurizedsteam operated plant employing the injection, under gravity, of coldwater contained in a tank which is superelevated with respect to saidplant, said plant comprising a steam headspace defined by a liquid levelduring normal operation, said depressurization system comprising:

a pipeline, connected to said steam headspace and forming a closed looparound said plant;

a condenser, placed below said liquid level, which has an inletcollector and an outlet collector;

a delivery pipe which connects said tank with said plant;

an ejector, on said pipeline, which has an entrance section, an exitsection and a throat;

a pressure relief pipe which connects said delivery pipe and the throatof said ejector;

said injection being triggered by a natural circulation of steam in saidclosed loop caused by the condensation of steam from the plant in saidcondenser during the occurrence of an accident, characterised in that itcomprises:

a first siphon-type hydraulic shutoff, on said pipeline and arrangedclose to the inlet collector of said condenser;

a second siphon-type hydraulic shutoff on said delivery pipe and placedclose to the pressurized steam operated plant;

said second siphon-type hydraulic shutoff being below said firstsiphon-type hydraulic shutoff.

The principal advantage of the depressurization system according to theinvention is its rapid triggering, which occurs independently of themagnitude of the damage from which the accident stems.

Further characteristics and advantages of the invention will emerge moreclearly from the description of an example of the actuation of thedepressurization system for pressurized plant, applied to a pressurizedlight-water nuclear reactor, given hereinbelow with reference to theappended drawings by way of non-limiting illustration.

In these drawings:

FIG. 1 represents diagrammatically a pressurized plant depressurizationsystem according to the invention; and

FIG. 2 represents diagrammatically the depressurization system of FIG. 1on the occurrence of an accident.

In the drawings a pressurized steam operated plant depressurizationsystem is indicated overall as 1.

Said plant consists, in this example of the actuation of the system 1according to the invention, of a thermonuclear plant, designed, forexample, to produce electrical energy. The drawings show a pressurizedvessel 2 containing the core, not shown in the drawings, of nuclear fuelfor the plant, a pressurizer 3 and a hot leg 4 of the pressurizedcircuit of said plant. The hot leg is connected to the pressurizedvessel 2 at an aperture 23 thereof.

The hot leg 4, in the region of said pressurizer 3, comprises a limb 35.

The aforementioned components 2, 3, 4 interact with the depressurizationsystem 1 and with one another, as will be described subsequently.

The depressurization system 1 comprises a main tank 5 filled, up to alevel 6, with cold water which, according to a preferred embodiment ofthe invention, contains, in solution, an element capable of inhibitingthe nuclear fission reaction, for example boron.

The level 6, for convenience of description, divides the main tank 5into an upper part 7 and a lower part 8.

In fact, the tank 5 may also be completely filled with water and thelevel 6 may be located inside pipes situated above the main tank 5 andcommunicating therewith.

The depressurization system 1 also comprises an ejector 9 and acondenser 10.

The ejector 9 has an entrance section 11, an exit section 12 and athroat 13.

The condenser 10, preferably of the type with straight tubes indicated30, has an inlet collector 14 and an outlet collector 15.

The thermonuclear plant comprises, inside the pressurizer 3, a hot watervolume 16 and a steam headspace 18 situated above said hot water volume16, defined by a liquid level 17.

The main tank 5 is superelevated with respect to the entire pressurizer3 and the ejector 9 which, in turn, is completely superelevated withrespect to the level 17 of the hot water volume 16 inside thepressurizer 3, whereas the pressurized vessel 2 and condenser 10 arecompletely below said level 17.

The depressurization system 1 comprises a first, a second and a thirdlink pipes indicated 19, 20, 21 respectively, and an injection pipe 22.

The first link pipe 19 is arranged between the steam headspace 18 andthe tank 5, at its upper part 7.

As a result, the main tank 5 and the pressurizer 3 are at the samepressure.

In the figures it may be observed how the level 6 can be situated insidethe first link pipe 19 through which, during normal operation of thereactor, a small quantity of condensed steam is received from thepressurizer 3. This small quantity is intended to drip into the maintank 5.

The second link pipe 20 is arranged between the first link pipe 19 andthe entrance section 11 of the ejector 9, branching off from a union 36,on the first pipe 19.

The third link pipe 21 is arranged between the exit section 12 of theejector 9 and the inlet collector 14 of the condenser 10, while theinjection pipe 22 is arranged between the outlet collector 15 and thepressurized steam operated plant, in this illustrative embodiment overthe cylindrical vessel 2, below its aperture 23.

In accordance with the present example of a preferred embodimentaccording to the invention, the pressurized steam operated plant,interacting with said depressurization system 1, and in particular saidthermonuclear plant, comprises a first and a second conduits, indicated24 and 25 respectively.

The first and second conduits 24, 25 hydraulically connect thepressurizer 3, in the region of its hot water volume 16, with the hotleg 4 of the thermonuclear plant, at its limb 35.

Since said limb 35 of the hot leg 4 is pipework having a considerablediameter, in particular the first conduit 24 is connected to the hot leg4 from below, that is, at its lower part, whilst the second conduit 25is connected to the hot leg 4 from above, that is, at its upper part.

The function of this particular arrangement of the first and secondconduits 24, 25, of the pressurized steam operated plant, will emerge inthe course of the description of the operation of the depressurizationsystem 1 in the event of an accident.

Because the condenser 10 is placed below the level 17 of the watervolume 16 inside the pressurizer 3, and because the circuit consistingof the succession of the third link pipe 21, the condenser 10, the firstinjection pipe 22, the hot leg 4 and the fourth link pipe 24 is open,the condenser 10 is completely flooded by water, from the pressurizer 3,which fills the whole of the aforesaid circuit up to a point, in thethird link pipe 21, located at the same height as said level 17.

The depressurization system 1 described above comprises a pipeline, madefrom the succession of the first, second and third link pipes 19, 20, 21and the injection pipe 22, which is connected to said steam headspace 18of the thermonuclear plant and which forms a closed loop around saidnuclear plant, i.e. between said steam headspace 18, contained in thepressurizer 3, and said pressurized vessel 2.

The depressurization system 1 furthermore comprises a delivery pipe 26which connects the lower part 8 of said tank 5, filled with cold,borated water, and said thermonuclear plant, in this illustrativeembodiment in the region of its cylindrical vessel 2.

The depressurization system 1 then comprises, on said delivery pipe 26,a siphon 27.

Said siphon 27 comprises a rising limb 27a, an upper limb 27b and afalling limb 27c which continues downwards.

The upper limb 27b is located above the level 6 of the cold watercontained in the main tank 5, so that the cold water from said main tank5 fills up the delivery pipe 26 to a point in the rising limb 27a,stationed at the same height as the level 6.

As stated earlier, the upper limb 27b of the siphon 27 is located abovethe first link pipe 19.

Below said level 6 the throat 13 of the ejector 9 is connected to saiddelivery pipe 26, at a point following the siphon 27 starting from themain tank 5, by a permanently open pressure relief pipe 28.

On the third link pipe 21 of said pipeline forming said closed looparound the thermonuclear plant, the system 1 comprises a firstsiphon-type hydraulic shutoff 32, close to the inlet collector 14 of thecondenser 10 and completely below the latter.

Said siphon-type hydraulic shutoff 32 in fact has a falling limb 32a, alower limb 32b and a rising limb 32c, following which the third linkpipe 21 feeds into the inlet collector 14 of the condenser 10.

Furthermore, following said pressure relief pipe 28, the system 1comprises, on the delivery pipe 26, a second siphon-type hydraulicshutoff 29 close to the pressurized vessel and placed completely belowthe condenser 10 and also below the lower limb 32b of the firstsiphon-type hydraulic shutoff 32.

Said second hydraulic shutoff 29 in fact has a falling limb 29a, a lowerlimb 29b and a rising limb 29c, following which the delivery pipe 26feeds into the pressurized vessel 2 via a sprayer 31.

The depressurization system 1 then comprises an auxiliary pipe 33 whichhydraulically connects the outlet collector 15 of the condenser 10 tothe pressurized vessel 2.

On the auxiliary pipe 33 the system 1 comprises a secondary tank 34which is completely superelevated with respect to the condenser 10 andis placed entirely below the level 17 of the water inside thepressurizer 3.

With reference in particular to FIG. 1, the liquid level 17 inside thepressurizer 3, which contains the steam headspace 18 of thethermonuclear plant, is produced during normal operation.

On the occurrence of an accident causing a loss of water content fromthe thermonuclear plant, the liquid level 17 falls inside thepressurizer 3 and also in the third link pipe 21.

On account of the presence of the first siphon-type hydraulic shutoff32, however, even when the liquid level 17 falls below the condenser 10and, even before that, below the secondary tank 34, these latter do notdrain, remaining completely swamped with water from the plant. This isbecause the hydraulic seal which is produced in the condenser 10, in thetank 34, in the injection pipe 22 and in the auxiliary pipe 33, is lessthan pressure weighing on the declining level 17 inside the pressurizer3 and the third link pipe 21.

When the liquid level 17 falls below the lower limb 32b of thesiphon-type hydraulic shutoff 32 (FIG. 2), there is drainage of thecondenser 10 through the injection pipe 22.

The hydraulic thrust which brings about said drainage is produced by thehydraulic seal determined by the difference in altitude between thedeclining liquid level 17, situated below the lower limb 32b of thefirst siphon-type hydraulic shutoff 32, and the lower limb 32b.

At this point the condenser 10 is swamped solely by the steam, from thethermonuclear plant, which condenses.

Hence, further water gathers in the outlet collector 15 and prevents thedrainage of the secondary tank 34 through the auxiliary pipe. This isbecause the auxiliary pipe 33 and the tank 34 constitute a kind ofsiphon, drainage of which is blocked by the water from the outletcollector 15.

The incondensible gases, which are always present in some quantity inpressurized steam operated plant and are present in particular inthermonuclear plants where they are produced by hydrolysis, collect inthe outlet collector 15.

When this happens the secondary tank 34 can drain through the auxiliarypipe 33 and the injection pipe 22 and receives all the incondensiblegases present in the condenser 10.

By virtue of the aforesaid provisions, the condenser 10 can operateimmediately at maximum capacity and rapidly condenses, inside the tubebundle 30, the steam that is present and this brings about a naturalcirculation of steam inside the pipeline consisting of the succession ofthe first, second and third link pipes 19, 20, 21 and the injection pipe22, which forms a closed loop around the thermonuclear plant, and hencealso through the ejector 9.

Said natural circulation of steam triggers the injection under gravityof the cold water contained in the superelevated main tank 5, in themanner which will be explained below.

The passage of the steam through the ejector 9 causes a reduction inpressure in its throat 13 which sucks the steam contained in thedelivery pipe 26 through the permanently open pressure relief pipe 28,thus also raising the water contained in the rising limb 27a of thesiphon, until it passes the upper limb 27b.

When this is reached the siphon 27 is triggered, the water from the tank5 being subjected to a hydraulic thrust caused by the difference inaltitude between the level 6 of the water inside the tank 5 and thedeclining liquid level 17, so that the water from the tank 5 isinjected, under simple gravity, through the delivery pipe 26 anddirectly into the pressurized vessel 2 where it is then sprayed by thesprayer 31.

This achieves rapid condensation of the steam present, bringing aboutdepressurization of the whole of the thermonuclear plant.

The pressure reduction in the throat 13 of the ejector 9 does notlikewise suck the steam contained in the pressurized vessel 2 throughthe sprayer 31 because of the presence of the second siphon-typehydraulic shutoff 29 which remains full of water even when the firsthydraulic shutoff 32 has drained, it being at a lower level.

When the accident which has occurred is, for example, a rupture oflimited cross section in the plant in the region of said steam headspace18, the steam which escapes from the thermonuclear plant comes,completely or in part, directly from the pressurized vessel 2 where itis generated on account of the nuclear decay reactions which produceresidual heat.

This steam, in order to reach the rupture, travels along the hot leg 4and enters at the bottom of the pressurizer.

This situation is brought about with ease if, on account of the internalproduction of steam due to the rise in temperature, the level of thewater inside the pressurized vessel 2 falls below the aperture 23 fromwhich the hot leg 4 branches off.

In this situation the hot leg 4 will contain a steam headspace and apart flooded with water originating from the hot water volume 16 of thepressurizer 3.

The flow of steam from the pressurized vessel 2 might then oppose thefall in the water contained in the pressurizer 3.

Such a situation could prejudice the correct operation of thedepressurization system 1 since the fall in the liquid level 17 could bedelayed with consequent non-opening of the first siphon-type hydraulicshutoff 32 which sets the entire system 1 into motion.

Furthermore, in the case of a thermonuclear plant, the risk would beincurred that the fuel elements contained in the cylindrical vessel 2would be uncovered of coolant, with devastating consequences.

This drawback finds a solution in the fact that the circulation of thesteam generated in the cylindrical vessel 2 finds an outlet through thesecond conduit 25 which, being connected to the hot leg 4 above thelatter at its limb 35, communicates directly with the steam headspacecontained in the hot leg 4.

In fact the first and second conduits 24, 25 constitute a loopedcircuit, which also comprises the limb 35, inside which is a portion offluid in the gaseous and lighter state, the steam in the second conduit25, and a portion of fluid in the liquid and heavier state, the waterfrom the first conduit 24.

The difference in density of the fluids in the various conduits 24, 25sets up a natural circulation which enables the steam to reach thepressurizer 3 and, simultaneously, enables the water from thepressurizer 3 to flow into the hot leg 4, through its limb 35, and fromthere to the pressurized vessel 2.

The depressurization system 1 described above can be subjected tomodifications and simplifications.

For example, with the second hydraulic shutoff 29 still present, theinjection pipe 22, the auxiliary pipe 33 and the delivery pipe 26 can beconnected directly to the hot leg 4 of the pressurized steam operatedplant.

Furthermore, the delivery pipe 26 upstream of the pressure relief pipe28 can be dispensed with, the third link pipe 21 and the injection pipe22 being provided as a path for the injection, under gravity, of coldwater from the tank 5, and a siphon-type hydraulic shutoff, similar tothat provided in the present illustrative embodiment on the deliverypipe 26, being arranged on the first injection pipe 22 at a level belowthat of the first hydraulic shutoff 32.

Moreover, the outflow of water from the tank 5 can be slowed down byarranging suitable head losses in the delivery pipe 26.

In addition to the advantage mentioned above the pressurized steamoperated plant depressurization system according to the invention isable to cope with any variety of accident which requires the rapiddepressurization of the plant.

In particular, by virtue of the design adopted for the pressurized steamoperated plant, the steam circulating in the depressurization plant isprevented from impeding the flows of hot or cold water which are inducedin the depressurization system according to the invention.

Furthermore, the triggering of the injection, under gravity, of watertakes place even in the presence of incondensible gases mixed in thesteam, that were previously dissolved in the water of the plant andsubsequently liberated by the reduction in pressure or produced thereinby hydrolysis.

Moreover, the present invention is suitable for a wide variety ofpressurized steam operated plants, including any nuclear plant operatedwith pressurized or boiling water and thus requiring the fitting ofcomponents of simple design.

In order to satisfy particular local contingent exigencies, a personskilled in the art will be able to make numerous variants to thedepressurization system according to the invention, all included,however, within the scope of protection of the invention, as defined bythe following claims.

We claim:
 1. Depressurization system (1) for pressurized steam operatedplant (2, 3, 4) employing the injection, under gravity, of cold watercontained in a tank (5) which is superelevated with respect to saidplant (2, 3, 4), said plant (2, 3, 4) comprising a steam headspace (18)defined by a liquid level (17) during normal operation, saiddepressurization system (1) comprising:a pipeline (19, 20, 21, 22),connected to said steam headspace (18) and forming a closed loop aroundsaid plant (2, 3, 4); a condenser (10), placed below said liquid level(17), which has an inlet collector (14) and an outlet collector (15); adelivery pipe (26) which connects said tank (5) with said plant (2, 3,4); an ejector (9), on said pipeline (19, 20, 21, 22), which has anentrance section (11), an exit section (12) and a throat (13); apressure relief pipe (28) which connects said delivery pipe (26) and thethroat (13) of said ejector (9);said injection being triggered by anatural circulation of steam in said closed loop caused by thecondensation of steam from the plant (2, 3, 4) in said condenser (10)during the occurrence of an accident, characterised in that itcomprises: a first siphon-type hydraulic shutoff (32), on said pipeline(19, 20, 21, 22) and arranged close to the inlet (14) of said condenser(10); a second siphon-type hydraulic shutoff (29) on said delivery pipe(26) and placed close to the pressurized steam operated plant (2, 3,4);said second siphon-type hydraulic shutoff (29) being below said firstsiphon-type hydraulic shutoff (32).
 2. Depressurization system (1)according to claim 1, comprising an auxiliary pipe (33), which connectsthe outlet collector (15) of the condenser (10) and the pressurizedsteam operated plant (2, 3, 4), and a further tank (34), completelysuperelevated with respect to the condenser (10) and placed entirelybelow said liquid level (17), on said auxiliary pipe (33). 3.Depressurization system (1) according to claim 1, wherein said pipelinecomprises:a first link pipe (19), between said steam headspace (18) andsaid tank (5); a second link pipe (20), between said first link pipe(19) and said entrance section (11) of said ejector (9); a third linkpipe (21), between said exit section (12) of the ejector (9) and saidinlet collector (14) of the condenser (10); an injection pipe (22),between said outlet collector (15) of the condenser (10) and thepressurized steam operated plant (2, 3, 4);said second siphon-typehydraulic shutoff (29) being arranged on said third link pipe (21). 4.Depressurization system (1) according to claim 1, comprising a siphon(27) on said delivery pipe (26), said siphon (27) comprising a risinglimb (27a), an upper limb (27b) and a failing limb (27c), said upperlimb (27b) being above the level (6) of the cold water contained in thesuperelevated tank (5).
 5. Depressurization system (1) according toclaim 1, wherein said delivery pipe (26) feeds into the pressurizedsteam operated plant (2, 3, 4) via a sprayer (31).
 6. A depressurizationsystem according to claim 1, wherein the pressurized plant furthercomprises a pressurized vessel, a pressurizer (3), a hot leg comprisingat least one limb (35), a first conduit (24) which connects saidpressurizer (3) with said hot leg (4) at said limb (35), a secondconduit (25) which connects said pressurizer (3), and said hot leg (4)at said limb (35), said first conduit (24) being connected to said hotleg (4) at its lower part, and said second conduit (25) being connectedto said hot leg (4) at its upper part.